1. Field of the Invention
This invention relates to diaphragm spring brake actuators of the type used with air brake systems on vehicles such as trucks. In one of its aspects, the invention relates to a spring brake actuator having a tamper-resistant spring chamber.
2. State of the Prior Art
Spring brake actuators are in common use with air brake systems used on trucks, buses, and towed vehicles. Such actuators are typically provided with a service chamber for normally applying and releasing the brakes in response to delivery and exhaust of compressed air, and a spring chamber disposed in tandem with the service chamber for providing parking or emergency brake functions. A spring brake actuator uses spring force to operate a service brake actuator and apply brakes when pressurized air in the spring chamber is reduced below some predetermined level. Air pressure may be reduced in the spring chamber to apply the brakes under the control of the operator or automatically as a result of failure of the pressurized air system. The service chamber and spring chamber are separated by an adapter or flange casing which forms a wall between the two chambers.
In a typical spring brake actuator, a barrel-shaped power spring is used to store energy and to exert the large force required for braking in the event of air pressure failure. Air pressure acting on a diaphragm or a piston is employed to compress the spring and maintain it in its brake release position. When the air is exhausted, the spring acts on the diaphragm, typically an elastomeric diaphragm or a piston, and through an actuating rod exerts the spring force on the service push rod to apply the brakes in the event of a failure of the system air pressure.
The spring brake actuator operates within the spring chamber, which is typically formed by clamping an elastomeric diaphragm between a head (sometimes also known as a spring housing or spring chamber) and the adapter. The power spring is typically compressed within the spring chamber between the head and the diaphragm. The spring has a high spring constant and typically weighs 3 pounds or more, being compressed to a linear length of less than 3 inches from an original uncompressed length in an extended condition of from 9 to 12 inches. With a high spring constant, the spring has a substantial amount of potential energy, exerting a force on the head of from 2,000 to 3,000 pounds.
In previous diaphragm style brake actuators, the brake actuator head is secured to the adapter by means of a band, generally U-shaped in cross section, to clamp mating flanges on the head and adapter with the diaphragm clamped therebetween. Typically the band is formed of sections bolted together for convenient disassembly. Because the power spring is under great pressure, means must be provided to restrain or "cage" the power spring before the spring chamber can be safely disassembled. Failure to properly cage the power spring prior to disassembly and the resulting sudden release of potential energy in the spring can cause the head and adapter to fly apart.
To deter disassembly of the spring chamber, the clamp band has been formed from a continuous ring, deformed over the flanges to form what is commonly termed a sealed brake. Safety is an advantage of a sealed brake. Because it must be deformed to be removed, a sealed brake clamp band or deformed flange deters disassembly of the spring chamber.
The same feature which makes sealed brakes safer also deters future repair, because disassembly is difficult without damaging the brake. If the diaphragm should fail for example, the entire brake actuator may need to be replaced. In any event, reconditioning of a sealed brake is a very difficult procedure.
Other means are used for securing the brake actuator head to the adapter in a tamper-resistant brake. For example, U.S. Pat. No. 5,315,918 to Pierce, issued May 31, 1994, discloses a bayonet mount for securing the brake actuator head to the adapter. The brake actuator head and the adapter have complementary axially-extending lips. One of the lips has a series of openings formed therein and in which is received a plug weld to secure the actuator head to the adapter.
U.S. Pat. No. 5,285,716 to Thompson, issued Feb. 15, 1994, discloses the welding of a brake actuator head to an adapter in such a manner so that the weld securing the brake actuator head to the adapter is a sufficient distance from the diaphragm so as not to burn the diaphragm. Specifically, the brake actuator head has an axially-extending flange that overlaps an axially-extending flange of the adapter and extends a sufficient distance from the diaphragm compressed between the actuator head and the adapter so that the heat from the weld formed at the end of the axially-extending flange of the actuator head will not burn the diaphragm.